Fake etcd endpoint speaking the etcd v2 HTTP/JSON API over TCP.
etcd keys are hierarchical paths rather than buckets, so this maps onto the
bucketless side of FakeRiak.SimpleStore. Supported routes:
GET /v2/keys/:key -> read a key
PUT /v2/keys/:key -> write a key (body: `value=...`)
DELETE /v2/keys/:key -> remove a key
GET /version | /healthJSON responses are rendered with the built-in JSON module.